1. Field of the Invention
One or more embodiments pertain to the field of motion capture and initialization of sensors associated therewith that may be coupled with a user or piece of equipment. More particularly, but not by way of limitation, one or more embodiments enable initializing an inertial sensor using soft constraints and penalty functions to estimate the initial conditions of an inertial sensor device. The initialization generally occurs at a point in time based on data captured from this device during an initialization time interval, and on a set of soft constraints on the motion of this device during this interval.
2. Description of the Related Art
Inertial sensors are commonly used for motion tracking and navigation. They offer the advantage that they can be used to estimate the motion of an object without the need for external inputs or references. These systems typically include accelerometers to measure linear forces, and gyroscopes to measure rotational motion.
Such systems have been used for many years as navigation aids for ships and airplanes. More recently as inexpensive MEMS sensors have become widely available, inertial sensors are increasingly embedded into consumer products such as smartphones.
Pure inertial sensors are unable to detect uniform linear motion; this is essentially a consequence of the principle of relativity. They are also unable to determine absolute position or orientation. Effectively inertial sensors can be used for tracking changes in position and orientation. As motion tracking or navigation devices, they rely on dead-reckoning techniques to estimate new positions and orientations from known starting conditions and from the changes detected by the inertial sensor.
Applications using inertial sensors must therefore use some method to determine the initial conditions of the sensor prior to measuring the changes in position and orientation via the inertial data. Existing systems generally use one or both of two approaches for this initialization.
One existing approach to inertial sensor initialization is to place the sensor in a known position and orientation, and to ensure that it is at rest (with no linear or rotational motion) for some period of time prior to beginning motion tracking. The initial position and orientation of the sensor can then be provided directly to the motion tracking algorithm, and the initial linear and angular velocity can be set to zero to reflect the at-rest initial state. This approach is extremely simple, but may not be applicable in some situations. Sometimes the initial position or initial orientation may not be known. In some applications it may not be feasible to require an at-rest period of time before beginning motion tracking.
Existing systems that require the sensor to be at rest prior to starting motion tracking can sometimes obtain an estimate of the initial tilt of the sensor relative to vertical, by using the accelerometer as a sensor of the gravity vector. This technique is only applicable when the sensor is at rest and when there is negligible vibration; otherwise the accelerometer data will reflect a combination of true acceleration and gravity.
A second existing approach to inertial sensor initialization is to supplement the inertial sensor data with other sensor data feeds. For example, a GPS system is often used in conjunction with an inertial sensor to provide a data feed with absolute position information. A magnet may be used in conjunction with an inertial sensor to provide a data feed with absolute orientation information. These types of hybrid systems combining inertial and non-inertial sensors are commonly used for navigation. For example, a GPS system may be used as a primary navigation tool, with an inertial sensor providing extrapolation between GPS updates and backup in case of temporary unavailability of a GPS signal.
The disadvantage of hybrid systems is the cost and complexity of the additional sensor hardware and software. In particular for GPS (or a similar location-finding system, such as one using cell phone signals), a GPS receiver must be integrated into the sensor hardware. This may be impractical, or it may not work at all in certain environments such as indoors.
In summary, known systems for initializing an inertial sensor generally rely on either placing the sensor at rest and in known conditions, or supplementing the inertial sensor with additional sensor feeds. Either approach has limitations and disadvantages for certain applications. There is therefore a need for an inertial sensor initialization method that can be used under more general conditions and without the need for additional sensor hardware.